1. Field of the Invention
The present invention relates to a composition suitable for sports equipment in general, and more particularly to a composition suitable for use in golf ball manufacture. In one embodiment, the novel composition of the present invention is used in the manufacture of a golf ball comprising a core, a cover layer and, optionally, one or more inner cover layers. In one preferred embodiment, a golf ball is disclosed in which the cover layer comprises the novel composition of the present invention. In another preferred embodiment, a golf ball is disclosed in which at least one intermediate layer comprises the novel composition of the present invention. In another preferred embodiment, a golf ball is disclosed in which the core comprises the novel composition of the present invention.
2. Description of Related Art
The application of synthetic polymer chemistry to the field of sports equipment has revolutionized the performance of athletes in many sports. One sport in which this is particularly true is golf, especially as relates to advances in golf ball performance and ease of manufacture. For instance, the earliest golf balls consisted of a leather cover filled with wet feathers. These “feathery” golf balls were subsequently replaced with a single piece golf ball made from “gutta percha,” a naturally occurring rubber-like material. In the early 1900's, the wound rubber ball was introduced, consisting of a solid rubber core around which rubber thread was tightly wound with a gutta percha cover.
More modern golf balls can be classified as one-piece, two-piece, and three-piece (also known as “wound”). One-piece balls are molded from a homogeneous mass of material with a dimple pattern molded therein. One-piece balls are inexpensive and very durable, but do not provide great distance because of relatively high spin and low velocity. Two-piece balls are made by molding a cover around a solid core. These are the most popular types of balls in use today. Two-piece balls typically have a hard “cut proof” cover which gives a durable as well as a longer distance ball, but also a ball with lower spin rates, which result in a decreased ability to control the ball.
Three-piece or wound balls are made by molding a cover about a wound core. The core is typically made of rubber and can be solid, semi-solid or have a liquid center. A wound core is prepared by winding a thin thread of elastic material about the center core. The wound core is then covered with a durable cover material. Wound balls are generally softer and provide more spin, resulting in increased control over the ball, but such balls typically travel shorter distances than a two-piece ball. As a result of their more complex construction, wound balls generally require a longer time to manufacture and are more expensive to produce than two-piece balls.
Golf ball covers are sometimes made from balata rubber which is favored by some players because the softness of the cover allows them to achieve spin rates sufficient to allow more precisely control of ball direction and distance, particularly on shorter approach shots. However, balata-covered balls are expensive and less durable as compared to the other covering materials. In particular, balata covered balls are subject to nicks or cuts which can detract from the ball's flight characteristics. In addition, the balata-covered balls, although exhibiting high spin and soft feel, often are deficient in terms of the velocity of the ball when it leaves the club face which in turn affects the distance the ball travels. This distance is directly related to the coefficient of restitution (“C.O.R.”) of the ball.
One way to measure the coefficient of restitution is to propel a ball at a given speed against a hard massive surface, and measure its incoming velocity and outgoing velocity. The coefficient of restitution is defined as the ratio of the outgoing velocity to incoming velocity of a rebounding ball and is expressed as a decimal. As a result, the coefficient of restitution can vary from zero to one, with one being equivalent to an elastic collision and zero being equivalent to an inelastic collision.
The coefficient of restitution of a one-piece golf ball is a function of the ball's composition. In a two-piece or a multi-layered golf ball, the coefficient of restitution is a function of the properties of the core, the cover and any additional layer. While there are no United States Golf Association (“USGA”) limitations on the coefficient of restitution values of a golf ball, the USGA requires that the golf ball cannot exceed an initial velocity of 255 feet/second. As a result, golf ball manufacturers generally seek to maximize the coefficient of restitution of a ball without violating the velocity limitation.
Accordingly, a variety of golf ball constructions have been developed in an attempt to provide spin rates and a feel approaching those of balata covered balls, while also providing a golf ball with a higher durability and overall distance. This has resulted in the emergence of balls, which have a solid rubber core, a cover, and one, or more so called intermediate layers, as well as the application of new materials to each of these components.
A material which has been often utilized in recent golf balls is the family of ionomer resins developed in the mid-1960's, by E.I. DuPont de Nemours and Co., and sold under the trademark SURLYN®. These ionomer resins have, to a large extent, replaced balata as a golf ball cover stock material. Preparation of such ionomers is well known, for example see U.S. Pat. No. 3,264,272 (the entire contents of which are herein incorporated by reference). Generally speaking, commercial ionomers consist of a polymer of a mono-olefin, e.g., an alkene, with an unsaturated mono- or dicarboxylic acids having 3 to 12 carbon atoms. An additional monomer in the form of a mono- or dicarboxylic acid ester may also be incorporated in the formulation as a so-called “softening comonomer.”
The incorporated carboxylic acid groups are then neutralized by a basic metal ion salt, to form the ionomer. The metal cations of the basic metal ion salt used for neutralization include Li+, Na+, K+, Zn2+, Ca2+, Co2+, Ni2+, Cu2+, Pb2+, and Mg2+, with the Li+, Na+, Ca2+, Zn2+, and Mg2+ being preferred. The basic metal ion salts include those of for example formic acid, acetic acid, nitric acid, and carbonic acid, hydrogen carbonate salts, oxides, hydroxides, and alkoxides.
The first commercially available ionomer resins contained up to 16 weight percent acrylic or methacrylic acid, although it was also well known at that time that, as a general rule, the hardness of these cover materials could be increased with increasing acid content. Hence, in Research Disclosure 29703, published in January 1989, DuPont disclosed ionomers based on ethylene/acrylic acid or ethylene/methacrylic acid containing acid contents of greater than 15 weight percent. In this same disclosure, DuPont also taught that such so called “high acid ionomers” had significantly improved stiffness and hardness and thus could be advantageously used in golf ball construction, when used either singly or in a blend with other ionomers.
More recently, high acid ionomers are typically defined as those ionomer resins with acrylic or methacrylic acid units present from 16 wt. % to about 35 wt. % in the polymer. Generally, such a high acid ionomer will have a flexural modulus from about 50,000 psi to about 125,000 psi.
Ionomer resins can further comprise a softening comonomer, which is typically present from about 10 wt. % to about 50 wt. % in the polymer. Such ionomers will have a flexural modulus from about 2,000 psi to about 10,000 psi, and are sometimes referred to as “soft” or “very low modulus” ionomers. Typical softening comonomers include n-butyl acrylate, iso-butyl acrylate, n-butyl methacrylate, methyl acrylate and methyl methacrylate.
Today, there are a wide variety of commercially available ionomer resins based both on copolymers of ethylene and (meth)acrylic acid or terpolymers of ethylene and (meth)acrylic acid and (meth)acrylate, all of which many of which are be used as a golf ball component. The properties of these ionomer resins can vary widely due to variations in acid content, softening comonomer content, the degree of neutralization, and the type of metal ion used in the neutralization. The full range commercially available typically includes ionomers of polymers of general formula, E/X/Y, where E is ethylene, X is a softening comonomer such as acrylate or methacrylate present in an amount of from 0 wt. % to about 50 wt. % of the polymer, and Y is acrylic or methacrylic acid present in an amount from about 5 wt. % to about 35 wt. % of the polymer, and wherein the acid moiety is neutralized from about 1% to about 90% to form an ionomer with a cation such as lithium, sodium, potassium, magnesium, calcium, barium, lead, tin, zinc or aluminum, or a combination of such cations.
More recent developments in the field have attempted to utilize the various types of high acid and/or highly neutralized ionomers, both singly and in blend compositions to optimize the often conflicting golf ball performance requirements of high C.O.R. and ball velocity, and cover durability, with the need for a ball to spin and have a so-called soft feel on shorter iron shots. However, the incorporation of more acid in the ionomer and/or increasing its degree of neutralization results in a material with increased polarity, and hence one which is often less compatible with other potential blend materials. Also increasing the acid content of the ionomer while increasing C.O.R. may render the ball too hard and brittle causing a loss of shot feel, control (i.e., the ability to spin the ball) and may render the cover too brittle and prone to premature failure. Finally, the incorporation of more acid in the ionomer and/or increasing its degree of neutralization typically results in an increase in melt viscosity which in turn greatly decreases the processability of these resins. Attempts to mediate these effects by adding softer terpolymeric ionomers to high acid ionomer compositions to adjust the hardness and improve the shot “feel” often result in concomitant loss of C.O.R. and hence distance.
In addition, various hard-soft ionomer blends, that is, mixtures of ionomer resins, which are significantly different in hardness and/or flexural modulus, have been attempted. For instance, U.S. Pat. No. 4,884,814 discloses the blending of various hard methacrylic based ionomer resins with similar or larger quantities of one or more “soft” ionomer methacrylic acid based ionomer resins (i.e., those ionomer resins having a hardness from about 25 to 40 as measured on the Shore D scale) to produce relatively low modulus golf ball cover compositions that are not only softer than the prior art hard ionomer covers but also exhibit a sufficient degree of durability for repetitive play. These relatively low modulus cover compositions were generally comprised of from about 25 to 70% of hard ionomer resins and from about 30 to 75% of soft ionomer resins.
Also, U.S. Pat. No. 5,324,783 discloses golf ball cover compositions comprising a blend of a relatively large amount, e.g., 70-90 wt. %, of hard ionomer resins with a relatively low amount, e.g., 10 to about 25-30 wt. %, of soft ionomers. The hard ionomers are sodium or zinc salts of a copolymer of an olefin having from 2 to 8 carbon atoms and an unsaturated monocarboxylic acid having from 3 to 8 carbon atoms. The soft ionomer is a sodium or a zinc salt of a terpolymer of an olefin having from 2 to 8 carbon atoms, methacrylic acid and an unsaturated monomer of the acrylate ester class having from 1 to 21 carbon atoms.
In order to further extend the range of properties of the ionomer resins to optimize golf ball performance, additional components have been added to them as “modifiers.” For example, U.S. Pat. No. 4,104,216 (Clampitt) discloses ionomers modified with 5-50 weight percent of a long chain (un)saturated fatty acid.
Also, Japanese Patent Application No. 48/70757 discloses ionomers modified with a high level of a low molecular weight saturated or unsaturated carboxylic acid or salt or anhydride, specifically 10 to 500 parts per 100 parts by weight of ionomer. The carboxylic acid may have 1 to 100 hydrocarbon carbon chain units. Stearic, citric, oleic and glutamic acid and/or salts are exemplified.
U.S. Pat. Nos. 5,312,857 and 5,306,760 disclose cover compositions for golf ball construction comprising mixtures of ionomer resins and 25-100 parts by weight of various fatty acid salts (i.e., metal stearates, metal oleates, metal palmitates, metal pelargonates, metal laurates, etc.).
U.S. Pat. No. 6,100,321 and U.S. Patent Publication No. 2003/0158312 A1, disclose ionomer compositions, which are modified with 25 to 100 parts by weight of a fatty acid salt such as a metal stearate, for the production of golf balls with good resilience and high softness. Unlike the earlier mentioned patents, which have employed metal stearates as a filler material, these patents disclose the use of relatively low levels of a stearic acid moiety, especially calcium stearate, to modify ionomers to produce improved resilience for a given level of hardness or PGA Compression values. The stearate-modified ionomers are taught as being especially useful when the ionomer is formulated for use as a golf ball core, center, one-piece ball, or as a soft golf ball cover.
Subsequent patent applications have furthered the use of such modified ionomers in golf ball covers. For example U.S. Pat. No. 6,329,458 is directed to a golf ball cover comprising an ionomer resin and a metal “soap,” e.g., calcium stearate. Finally, U.S. Pat. No. 6,616,552 discloses a golf ball including a multi-layer covet, one layer of which includes a heated mixture of an ionomer resin and a metal salt of a fatty acid, e.g., calcium stearate.
In addition to modifying ionomers by adding fatty acids and their metal salts, a number of patents have described modification of the unneutralized ethylene/carboxylic acid polymer, although not for use in golf balls. For instance U.S. Pat. Nos. 3,388,186 and 3,465,059 disclose compositions made by grafting amino acids or lactams onto a backbone chain containing recurring ethylenic units and reactive sites, such as carboxyl radicals of acid, ester or salt groups, and particularly acrylic acid esters.
U.S. Pat. No. 3,634,543 discloses nucleated graft polymers of polycaprolactam on carboxy-containing copolymeric backbones. The graft polymer is prepared by polymerization of caprolactam in the presence of a copolymer of an olefin and an unsaturated carboxylic acid.
U.S. Pat. No. 4,035,438 discloses an impact resistant mixture of polyethylene; a graft polymer of an ethylene/acrylic acid copolymer or ethylene/methacrylic acid copolymer as a graft substrate and grafted polymerized units of polycaprolactam.
Finally, U.S. Pat. No. 5,130,372 discloses an ionomer salt of low molecular weight reaction product of 1) a copolymer of an alpha-olefin and an unsaturated carboxylic acid; 2) at least one amino acid compound; and 3) a cation containing material. The unsaturated carboxylic acid is preferably an α,β-ethylenically unsaturated carboxylic acid. The ionomer has excellent compatibility with other polymers, particularly polyamides. The ionomer was described as useful in a method to flush water from pigments.
However, there remains a need for new materials with equivalent or improved properties to the ionomer resins of the prior art for use in golf ball manufacture, but which but which are not plasticized in the sense of reduced modulus and stiffness. There also remains a need for new materials, which are more compatible with other resins, and which also do not give a hard feel to the golf ball or render it brittle and prone to failure and which do not require addition of softer terpolymeric ionomers which can cause a loss of C.O.R. It would also be highly advantageous if such new materials would exhibit increased C.O.R. and modulus, and still be easily processable by having a low melt viscosity.